1. Field of the Invention
This invention relates to a curable aqueous coating composition which can form a coating having high hardness and excellent water resistance.
2. Description of the Prior Art
In general, curable water-soluble acrylic resin coating compositions are prepared by adding an unsaturated carboxylic acid to monomer components, copolymerizing them to obtain a resin, converting the resin into an amine salt thereof to render the resin water-soluble and then mixing the salt of the resin with a water-soluble aminoplast resin, such as a methylol-melamine resin and methylol-urea resin which are made using formaldehyde. When a coating composition of this type is subjected to baking, a condensation reaction takes place between hydroxyl groups or carboxyl groups in the copolymer and the water-soluble aminoplast resin to form a three-dimensionally cured coating film which is excellent in hardness, water resistance and solvent resistance. However, these compositions are disadvantageous in that large amounts of formaldehyde are released on curing by heating.